The invention relates to a buckle device for adjusting and clamping a strap, said buckle being composed of a pair of superposed rings presenting rectangular frames of different dimensions, the larger first ring comprising a first rectangular opening located under a second rectangular opening arranged in the second ring of smaller size so as to define a transverse slot for passage of the strap, the slot being arranged between one of the outer edges of the second ring, and the adjacent side of the first opening of the first ring, the buckle being located either in a blocking position of the strap in the slot following tensioning, or in a releasing position after a relative movement between the two rings resulting in an increase of the passage area in the slot.
Such a buckle is in particular used for securing a safety sit harness strap.